President Trump said a primary beef with House Democrats’ new $3 trillion coronavirus rescue package lies in the bill’s expanded vote-by-mail provisions and that Republicans would become ill at the mere thought of committing voter fraud.

“Voting by mail is very dangerous because people can grab ‘em, people can take that proxy, sign a name, they can do all sorts of things,” Mr. Trump said in an interview that aired Thursday on Fox Business Network.

“The level of dishonesty with Democrat voting is unbelievable,” he said. “If you told a Republican to vote twice, they’d get sick at even the thought of it, and you have people that vote numerous times. What’s happening is crazy.”

“So now they want to send out vote by mail — who knows who’s signing this stuff?” he said.

Mr. Trump pointed to California, which allows a practice known as “ballot harvesting” in which volunteers can collect other people’s absentee ballots, as a prime example.

Rep. Devin Nunes said on Wednesday that Democrats have forced the GOP to adopt the tactic or get left behind.

“Even though we don’t like it, we don’t want to, but we are forced to have to ballot harvest because it’s the only way to win,” the California Republican said on Fox News.

Republican Mike Garcia defeated Democrat Christy Smith in a special congressional election in California this week that was conducted largely by mail.

“As long as we have a robust ballot harvesting operation in November — and I hate saying that because it’s illegal in 49 states — I think Mike Garcia is in good shape to hold onto that seat,” Mr. Nunes said.

In addition to billions more in coronavirus aid, the House Democrats’ bill would expand access to voting by mail for federal elections.

Democrats said the move is necessary to prevent people from being forced to physically go to the polls amid the coronavirus pandemic.

“You know why I voted? Because I happened to be in the White House, and I won’t be able to go to Florida and vote,” the president said last month.

Members of the military also routinely vote absentee by mail, and a handful of states have already moved to voting virtually 100% by mail.

North Carolina had to hold a special election for the state’s 9th Congressional District seat last year. The 2018 results were thrown out after a Republican operative was charged with illegally collecting absentee ballots on behalf of the GOP candidate.